PharmPK Discussion - Creatinine Clearance in Amputees

On 2 Feb 2002 at 17:24:29, "Raymond A. Smith" (smigger.-at-.inetnebr.com) sent the message

The Cockroft-Gault equation is widely used in PK for estimation of CrClbased on SCr and body weight. Amputees, especailly lower limb, have adecreased muscle mass ( hence a potentially decreased SCr) and anincreased body weight. These two combined could result in anoverestimation of ClCr. One solution would be to use ideal or adjustedbody weight, based on patients height before amputation. Anyone have anyclinical experience with this situation?

I won't offer any empirical clinical experience but will suggest sometheoretical scientific rationale. Amputation and resultant loss ofmuscle mass would be expected to decrease creatinine production rate(CPR) (and all other things being equal a decrease in Scr). C&G andother empirical formulae for CLcr are actually predicting CPR. With theassumption that Scr is at steady state then CLcr can be predicted fromCPR/Scr.

Amputation per se would *decrease* body weight but physical inactivityand other factors may indeed lead to increased adipose mass (which doesnot affect CPR) but may lead to *increased* body weight. Some amputeesmay increase their body weight by a net increase in muscle mass, andthus CPR, by athletic training and upper body building.

>These two combined could result in an>overestimation of ClCr. One solution would be to use ideal or adjustedbody weight, based on patients height before amputation. Anyone have anyclinical experience with this situation?

You should be trying to estimate post-amputation muscle mass.Pre-amputation ideal body weight predicted from height and sex would bea starting point and then one might deduct say 15% or so depending onthe extent of the amputation. Upper body builders may need an increasein predicted CPR because of a net increase in muscle mass.

Nick

Some discussion on this issue can be found here: Holford NHG. TheQuinidine-Digoxin Interaction. N. Eng. J. Med. 302: 864 (1980)